1. Field of the Invention
This invention relates to a laser printer, and more particularly to such a printer which is adapted to be used for color imaging.
2. Description of the Prior Art
Laser printers are used in photography and in the graphic arts for printing on a receiving medium such as film. When such printers are used for color imaging, they generally include a separate channel for each of the primary colors. In U.S. Pat. No. 4,728,965, for example, there is disclosed a laser printer which includes three optical channels, and each channel includes a gas laser which projects a beam of intense coherent light at a predetermined wavelength. The intensity of the light beam in each channel is modulated by an acoustooptic modulator in accordance with an electrical signal representing image information for one of the primary colors. The three beams of light are combined by a beam combiner, and the combined light beam is scanned onto a receiving medium by a rotating polygon.
In recent years, there have been attempts to use diode lasers instead of gas lasers in laser printers. The use of diode lasers reduces the cost of the printer and permits a drastic reduction in the size and complexity of the printer. Diode lasers can be modulated directly at frequencies as high as several hundred MHz by simply modulating the drive current, and thus, no external modulators are required. Further, the low intrinsic noise of diode lasers makes it possible to eliminate the high bandwidth servo controls used for noise cancellation in gas laser printers. There are, however, problems which have to be solved in using diode lasers in printers for the graphics arts. One of the problems is that available gallium aluminum arsenide diode lasers emit light in the infrared. As a result, a special recording material which is sensitive to the infrared must be used, and the diode lasers in a multilaser system must be carefully selected to obtain the widest possible spectral separation between the lasers. There are also problems in superimposing the diode laser beams in color systems to form a combined beam having the desired polarization and in shaping the combined beam to obtain high-quality, continuous tone images.